1. Field of the Invention
The present invention relates to heat exchanger devices, and, more particularly, to a rotary regenerative air heater or economizer which is mounted coaxially inside a flue duct, having a stack of heater lamellae packages which are alternatingly in contact with the hot flue gas and with the counter-flowing cold air which is to be heated.
2. Description of the Prior Art
Rotary regenerative air heaters which are arranged coaxially inside a flue duct are known from the prior art. They consist generally of a cylindrical stack of heater lamellae packages occupying the cross section of an enlarged length portion of the flue duct, so that the flue gasses have to pass axially through the lamellae packages. On the two axial sides of this heat exchanger stack are arranged air hoods which cover axially aligned, sector-shaped portions of the heat exchanger stack. A coaxially extending air duct supplies air through one air hood, removing it through the other.
A rotational relative motion between the cylindrical heat exchanger stack and the sector-shaped air ducts is obtained by either rotating the heat exchanger stack between stationary air ducts, or by rotating the two air ducts over the axial intake and outlet faces of a stationary heat exchanger stack. In both cases, the rotational relative motion results in alternating flows of hot flue gas and counter-flowing cold air through the lamellae packages, thereby transferring heat from the exiting flue gas to the incoming air. For average heat load conditions, the heater lamellae packages are composed of non-alloyed undulated sheet metal panels. For conditions of high heat load, these panels may be fabricated of non-scaling alloys. Air heater elements which operate at temperatures near the dew point may have enamelled heater lamellae.
It is also known from the prior art to be advantageous, from a point of view of fabrication costs and ease of assembly, to construct the heat exchanger stack of a succession of heat exchanger layers which consist of adjoining heater lamellae packages. This type of layered construction makes it possible to utilize different materials for different layers of heater lamellae packages, depending on whether they are on the hot or cold side of the heat exchanger stack. On the hot side, it may be necessary to use non-scaling sheet metal; on the cold side, it may be necessary to use sheet metal panels which are protected against corrosion.
It is further known to equip this type of rotary regenerative air heater with so-called soot blowing device, for the purpose of removing from the heater lamellae packages any soot and ash particles that may have been deposited by the passing flue gas. Such a soot blowing device normally operates with steam or compressed air, sweeping over the end faces of the heat exchanger stack in such a way that its blow jets move over the entire surface of the heat exchanger, once during each blowing cycle. A problem related to this type of soot blowing device is that the intensity of the blowing action must be kept within limits, in order to avoid erosion damage or vibration in the undulated sheet metal panels of the heater lamellae packages. On the other hand, these blow jets must be strong enough to pass through the successive layers of the heat exchanger stack at a speed which dislodges and removes the soot and ash deposits.
It is further known to operate the soot blowing device in such a way that it initially produces a counter-flow to the flue gas, whereupon it is reversed to the same direction as the flue gas, so that the loosened particles can be carried away by the exiting flue gas. However, there are certain limitations to this soot blowing approach: It can be used successfully only when the total height of the multi-layer heater stack is kept within certain design limits. If they are exceeded, in terms of the size of the heater exchanger stack and/or in terms of the number of layers which make up the complete stack, then it may happen that the deposits, instead of being removed, are compressed, especially in those heater lamellae packages which make up the middle layers of the stack. The result is a progressive clogging of the heat exchanger necessitating a shutdown of the entire boiler installation, to allow for the purging of the heater lamellae packages of the air heater with a high-pressure water jet.